1. Technical Field
This invention relates to reflective type, bright display elements useful as a display in wrist watches, meter display panels of vehicles, information display terminals for electronic memorandum books or computer notebooks, computers and televisions, or information bulletin boards, and to the construction and configuration of electronic apparatus employing such display elements as provided in a visual display, such as a LCD panel.
2. Background
Various kinds and styles of wrist watches or time pieces, such as analog-type of watches with hand pointers and digital-type of watches equipped with liquid crystal display elements, have been developed and offered as time-keeping products. Recently, in addition to these standard types of watches, hybrid-type of watches have also been developed which are equipped with a smaller LC (liquid crystal) window for display because of other types of information to be displayed. Also, there has been developed the two-layer type watches with a liquid crystal display element superposed on the surface of analog-type watch having hand pointers for indicating time. As examples, see Japanese Laid-Open Patent Application Nos. 94940/1979, 136718/1979, 26986/1980, 46845/1980, 188786/1980, 23024/1981 and 65618/1981. In the field of watches and electronic memorandum books, there have recently been developed various kinds of multi-functional type of watches capable of carrying out different kinds of informational functions. For these multi-functional uses in connection with watches, it is expected that the development of novel type of display elements may become the new practice and trend in future watch design.
On the other hand, displays employed in electronic apparatus for specialized display use, such as a meter panel for an automobile, are needed to display various kinds of information within a limited space. Furthermore, particularly in automotive meter panels, the display mode to be employed should be selected to be either analog type or digital type.
In addition, in the field of information devices including watches, the current trend has been-toward a more compact and portable type of structure with concurrent demand for reducing electric energy consumption of the display element to be mounted in these devices. In such portable information display devices or information processing apparatus, batteries have been conventionally employed. Therefore, the lifetime of batteries to be used is an important factor in their use. From this viewpoint of energy saving, devices incorporating a solar battery have recently been developed. In an apparatus disclosed in Japanese Patent Publication No. 61930/1991, for example, a liquid crystal display element and a solar battery are incorporated together in order to provide saving of space in the apparatus. In Japanese Laid-Open Patent Application No. 106725/1988, for example, there is disclosed an apparatus having a structure wherein a solar battery is designed to surround a LC display element.
Practical embodiments of such an apparatus employ liquid crystal material dispersed in polymer, which is then encapsulated in the liquid crystal display element. These encapsulated mediums are referred to as Polymer Dispersed Liquid Crystal ("PDLC") mediums. These types of mediums provide for a bright, reflective-type display element requiring no polarizing plates suitable for the use in these types of liquid crystal display elements. PDLC display elements are of two modes. One mode is the type of medium that becomes transparent to transmissive light in the presence of an applied electric field and scatters light in the absence of an applied electric field, as illustrated in Japanese Patent Publication No. 501631/1983. Another mode, which is referred to as "reverse PDLC", is the type that scatters light in the presence of an applied electric field and absorbs light or becomes transparent to light in the absence of a field, as illustrated in Japanese Laid-Open Patent Application No. 227684/1992 relating to a gel network type of reverse PDLC, Japanese Laid-Open Patent Application No. 119302/1993 relating to particle-orientation/dispersion type of reverse PDLC, and U.S. Pat. No. 4,994,204 relating to a liquid crystal droplet-dispersion type of reverse PDLC. For the purpose of increasing birefringence of liquid crystals and polymers of these PDLC modes as well as improving their degree of light scattering, liquid crystals or polymers having a terphenyl skeleton or a terphenyl skeleton containing a hetero atom are employed.
Conventional hybrid-type watches may be provided with separate analog display and digital display portions and, therefore, individual display areas are reduced in size to accommodate spacing rendering it more difficult to see the displayed information. In addition, since the digital display portion is of the twisted nematic type employing a polarizing plate, the displayed image appears darker, further decreasing its visibility. Furthermore, the incorporation of a solar battery in such a hybrid-type watch, while desirable, is more difficult due to limitations on available space. A solar battery lias been previously incorporated with a liquid crystal display element in such hybrid-type watches in the past. However, the liquid crystal display element incorporated with such a solar battery provides for low transmittance levels. For example, a twisted nematic type liquid crystal using a polarizing plate had a transmittance of 30% or less and a guest-host type liquid crystal had a transmittance of 60% or less, resulting in a comparatively lower production efficiency of electric power of the solar battery. Also, since the display mode employs a dark reflector as a background, such as the solar battery itself, the visibility of the watch display becomes even more reduced. In addition, in case where a liquid crystal display element utilizing dynamic scattering effect is superposed on a solar battery, the display principle accompanies ionic flow, and, therefore, requires an increase in electric power consumption of the battery. From these reasons, the integration of a liquid crystal display element with a solar battery has not been, in practice, successful.
In the case where the display portion of electronic apparatus has a special utility, such as an on-vehicle-type meter display panel, both an analog and digital display are sometimes incorporated into a single, limited display space. In this situation, it is necessary to provide a larger display space.
Recently, car navigation systems, on-vehicle television receivers and video projectors have come into use. In general, the displays of these electronic apparatus are placed near the central region of the vehicle console section. As a result, the vehicle driver must turn his head to align his/her eyes toward the passenger's seat in order to observe the display, which may be dangerous since the eyes are not momentarily observing the roadway.
Also, conventional on-vehicle-type meter display panels and the like employ a self-emissive light system or a backlight system to render the displayed information more visible. This results in the disadvantage that the display of the panel can not be seen well in the daytime, particularly when the outside ambient light is very bright. In addition, users may desire to select either an analog or digital type of display according to their individual taste or desires.
Conventional PDLC or reverse PDLC compounds used in PDLC display elements have a fluorescence emission in the visible region, such as compounds having a terphenyl skeleton, for purposes of improving birefringence or for mismatching of the refractive indices between the employed liquid crystal material and the polymer. As a result, noticeable fluorescence and haze in the transparent state is produced. Therefore, a good transparent state in the display system can not be created so that the display contrast is reduced. For example, in a PDLC display element or panel, the transparency in the presence of an applied voltage becomes low. When a light absorbing layer is placed at the back surface of the element, the color of the background appears turbid due to the fluorescence and haze caused by this particular kind of liquid crystal layer.
Moreover, in the case of a reverse PDLC display element or panel, because of the fluorescence and haze of this kind of liquid crystal layer, the degradation of the transparent state in the absence of an applied voltage results so that when a light absorbing layer is placed at the back surface of the element, the color of the background also appears turbid due to the fluorescence and haze created by the liquid crystal layer. Also, a reverse PDLC display element containing a dichroic dye mixed with the liquid crystal and including a light reflective layer positioned at the back surface provides the problem that the color state with the dichroic dye appears turbid in the absence of applied electric field due to fluorescence and haze created by the liquid crystal layer, resulting in degradation of the display contrast.
In the employment of conventional techniques for the purpose of improving scattering degree in the scattering mode of the display, a low molecular weight, chiral component is placed in the liquid crystal medium. During the preparation process, a polymer precursor incorporated with the liquid crystal compound is polymerized to increase the proportional content of the low molecular weight, chiral component in the liquid crystal. The increase in the proportional content of the low molecular weight, chiral component reduces the chiral pitch in the liquid crystal. As the result, a mismatch between the twist structure of the polymer at the initial stage of polymerization as compared to after the polymerization is produced, resulting in creation of haze in the display element in a transparent state in the absence of an applied electric field. Furthermore, such a change in the chiral pitch caused by a low molecular weight, chiral component is temperature dependent, which causes changes, particularly increases, in the amount of display haze.
The creation of haze, therefore, is a serious problem in a transparent-scattering, switching mode PDLC medium containing no dichroic dye. For example, in the case used as a cover glass and a clockface of a watch, a haze generated in a transparent state of a display element makes the production value remarkably deteriorate. In case of a display element containing dichroic dye with a reflective layer on its back surface, the display element employs the modes of switching between light absorption and light scattering. However, even in this case, the creation of the fluorescence and haze effect also remarkably deteriorates contrast in the liht absorbing state of the display.
Accordingly, it is an object of this invention to provide a novel PDLC or reverse PDLC display element having brighter display quality, particularly in its light scattering state, compared to previously known PDLC and reverse PDLC display elements, as well as providing less fluorescence and haze effect in its transparent state while improving overall display contrast. Moreover, the liquid crystal/polymer medium is provided to have high reliability, enhancing the reliability of the display element so that different active elements or a color filter may be effectively employed to provide a display element having multiple utility in various different types of electronic apparatus.
Another object of this invention is the provision of an electronic apparatus having lower power consumption with improved display quality and life by combining a reverse PDLC display element which is highly light transmissive so as to be employed with any one of a plurality of different types of electronic apparatus having its own display apparatus and/or with an underlying solar battery.
A further object of this invention is provision of a technique for improving the visibility of a display in an electronic apparatus, particularly in bright ambient light.